Method and apparatus for eliminating cavitation



Nov. 10, 1970 D. M. FRAZIER 3,539,275

METHOD AND APPARATUS FOR ELIMINATING CAVITATION Filed Aug. 7, 1968 04 via M Frazier INVENTOR.

G v 'IIIIIIIII BY 1 United States Patent 3,539,275 METHOD AND APPARATUS FOR ELIMINATING CAVITATION David M. Frazier, 208 Shorecrest, Tampa, Fla. 33609 Filed Aug. 7, 1968, Ser. No. 750,804 Int. Cl. FMf 5/10, 5/44, 5/48 US. Cl. 417-189 8 Claims ABSTRACT OF THE DISCLGSURE This invention relates to a means for the reduction of cavitation caused by a liquid jet stream, and more particularly to a method and apparatus for reducing cavitation in a jet pump by introducing a gas of regulated pressure in the vicinity of the discharge nozzle of the primary liquid.

Jet pumps for pumping liquid or solid-liquid mixtures, usually called, slurries, are known. Such a pump is shown in US. Pat. 3,389,938 issued to D. M. Frazier. They utilize a high-pressure liquid jet stream to both drive and draw the slurry along its transporting conduit. The slurry is both driven by the jet stream which impinges on it and simultaneously drawn by the vacuum created behind the jet stream.

Unfortunately, liquids will vaporize at a given temperature and pressure. The pressure level is commonly referred to as the vapor pressure of the liquid.

It is known that a high pressure, high velocity jet stream is capable of creating a vacuum which is below the vapor pressure of many liquids. In present jet pumps, when a jet stream of water is introduced into the jet pump at high velocity, there are localized areas near the vena contracta of the jet stream where the Water will vaporize, or cavitate. This cavity collapses as the flow continues downstream at a point of higher pressure. This causes energy losses within the jet pump, limits the efficiency of the pump, and in many instances, causes extreme wear and erosion of the pump body at the point where the vapor condenses and the liquid strikes the walls of the pump. Wear is frequently so bad that a jet pump pumping slurry performs satisfactorily for only a few hours before replacement of the pump or a nozzle adjustment is required.

It is an object of this invention to provide novel means for offsetting cavitation of a liquid stream.

Another object is to provide a novel method and apparatus for preventing such cavitation to reduce jet pump wear.

Still another object is to provide means for increasing jet pump efficiency, by eliminating the energy losses associated with cavitation.

A further object is to provide means for controlling the pressures within the jet pump in the vicinity of the driving jet stream.

A still further object is to provide means whereby discharge pressures may be increased by reducing the losses due to cavitation, for these losses become exponentially greater with increasing jet velocities.

Another object is to provide a control over the velocity, pressure of density of the discharge of a jet pump and also the quantity of secondary liquid which flows through the pump.

Other objects of the invention will in part be obvious and will in part appear hereinafter.

3,539,275 Patented Nov. 10, 1970 It has now been found that by applying a gas, preferably non-condensable, to the vena contracta of the jet stream, prevents cavitation. The gas is preferably supplied at a pressure just above the vapor pressure of the liquid so as to have the least effect on the suction action of the jet pump on the liquid or slurry being pumped. This is easiest accomplished by an open pipe to the atmosphere with a pressure regulator controlled by a pressure sensor in communication with a plenum which feeds gas to the vena contracta.

The invention accordingly comprises the several steps and the relation of one or more of such steps with respect to each of the others and the apparatus embodying features of construction, combination of elements and arrangement of parts which are adapted to effect such steps, all as exemplified in the following detailed disclosure, and the scope of the invention will be indicated in the claims.

For a fuller understanding of the nature and objects of the invention, reference should be had to the following detailed description taken in connection with the accompanying drawings in which:

FIG. 1 is a cross-sectional side view of a jet pump having an annular nozzle structure in combination with the anti-cavitation means.

FIG. 2 is a cross-sectional elevation view of a jet pump having a peripheral nozzle structure in combination with the anti-cavitation means.

FIG. 3 is a partial cross-sectional elevation view of a jet pump having a centrally orientated nozzle in combination with the anti-cavitation means.

Referring now to the drawings and in particular FIG. 1, there is shown an annular-type jet pump 10. The pump consists of a cylindrical inlet conduit 12 for receiving the liquid or slurry to be pumped, and a cylindrical discharge conduit 14 for discharge of the pumped liquid or slurry. Afiixed in concentric alignment with said conduits is an outer cylindrical casing 16 having a high pressure primary liquid plenum 18 and a gas plenum 20 separated by partition 22.

Primary liquid plenum 18 is in communication with a supply pipe 24 for receipt of high pressure primary liquid. The primary liquid flows from the plenum 18 into discharge conduit 14 through an annular nozzle 26 formed by an opening between the outer surface of inlet conduit 12 and the inner surface of discharge conduit 14 at the juncture of both. The nozzle produces a jet stream flow pattern as shown. The lowest pressure produced by the jet stream is at the vena contracta 28 which is located downstream of the nozzle approximately /2 the nozz e opening.

Gas plenum 20 communicates with a gas or air supply pipe 30 having a pressure regulating valve 32 located therein. If gas is used, it is preferably non-condensable. Pressure regulating valve 32 is controlled by a sensing means 34 which senses the pressure in plenum 20. Gas from the plenum 20 passes through a peripheral slot 36 formed in discharge conduit 14 adjacent its interior end. The slot is so positioned that gas flow into the conduit is in the vicinity of the vena contracta 28 of the jet stream issuing from nozzle 26.

In operation, the jet pump moves liquid or slurry through conduits 12 and 14 by high pressure primary liquid which is received from liquid plenum 18 through supply pipe 24. The incoming liquid displaces water through nozzle 26 where it emerges at a high velocity as a jet stream into conduit 14. The high velocity jet stream impinges on the liquid or slurry in said conduit and drives it forward. At the same time due to natural phenomena, pressure in the vicinity of the jet stream is lowered, particularly in the vicinity of the vena contracta 28. This low pressure area creates a vacuum resulting in a suction effect on the liquid or slurry in conduit 12. The suction effectively pulls the liquid or slurry from conduit 12 into conduit 14 and into contact with the emerging jet stream. In this manner, the liquid or slurry is continuously driven forward and discharged.

As the jet stream emerges from the nozzle, the pressure in the vicinity of the vena contracta 28 of the stream, is normally lower than the vapor pressure of the liquid of the jet stream. The vena contracta, as mentioned earlier, is located downstream from the nozzle approximately /2 the nozzle opening or within 3 diameters of the nozzle opening. Due to the lower pressure, the liquid at this point flashes into gas cavitating in the area designated 38. As the gas moves downstream it condenses into a liquid at point 40, a point of higher pressure. Severe erosion of the conduit 14 adjacent point 40, where the gas condenses and liquid strikes the interior wall of the conduit, usually occurs.

To eliminate this, a gas, such as air, is introduced into the area 38 via inlet supply pipe 30, plenum 20 and slot 36. The gas is introduced into area 38 at a pressure above the vapor pressure of the liquid to prevent it, the liquid, from flashing into gas.

The greatest pressure at which this gas will be introduced, is controlled by the suction created at the vena contracta 28. The upper limit of pressure flowing from slot 36, however, should be adjusted because the greater the pressure, the less suction will be generated on the liquid or slurry being pumped. It is therefore advantageous to introduce the gas at the lowest possible pressure just above the vapor pressure of the liquid in the jet stream, since it is at this pressure that the maximum suction will be retained to induce a pull on the liquid or slurry in conduit 12. Maximum pump performance is thereby attained.

An optimum pressure of gas introduced when the liquid in the jet stream is water would be approximately one pound per square inch absolute, which is above the vapor pressure of water, but which allows a 13.7 pound per square inch suction to be applied to the liquid in conduit 12, assuming the gas being introduced is via pipe 30 which is freely open to the atmosphere at approximately 14.7 pounds per square inch absolute pressure. The air moves through the pressure regulating valve 32 set at 1 p.s.i. absolute and thence into the plenum chamber Where a vacuum exists due to the action of the jet stream moving through nozzle 20. By sensing the pressure in plenum 20 through a suitable pressure sensing means 34 the pressure regulating valve can meter the air so as to maintain a flow into the pump at the desired pressure, in this instance 1 p.s.i. absolute. It is to be understood that instead of the above pressure regulating system for the gas, the gas may be effectively metered by providing a fixed orifice in lieu of slot 36 and where the size of the orifice determines the pressure of the gas.

A modification of the pump is shown in FIG. 2. In this modification the jet pump is provided with peripheral nozzles 42 in lieu of the annular nozzle 36 of FIG. 1. In this instance a continuous cylindrical conduit 44 is used to pump liquid or slurry. The nozzles are preferably formed by inclined bores cut through the wall of conduit 44. The bores are inclined in the direction toward the discharge end of the pump.

As in the first embodiment, affixed in concentric alignment with the conduit is an outer cylindrical casing 46 having a high pressure primary liquid plenum 48 and a gas plenum 50 separated by a partition 52.

Primary liquid plenum 48 is in communication with a supply pipe 54 for receipt of high pressure primary liquid. The primary liquid is discharged from plenum 48 into conduit 44 through nozzle 42. The liquid emerges as a jet stream in the conduit.

The gas plenum 50 also, as in the first embodiment, receives a gas through a supply pipe 56 having a pressure regulator valve 58 located therein. The valve is controlled by a sensing means 60 which senses the pressure in gas plenum 50. Gas from plenum 50 passes through a transverse bore 62 which is in communication with the bore of nozzle 42 adjacent its anterior opening into the conduit.

The operation of the pump is essentially as described for the first embodiment. To avoid repetition, a succinct description of the operation will only be given.

A slurry to be pumped is received in conduit 44. A liquid jet stream emerging from nozzle 42 impinges on the slurry driving it forward and simultaneously a vacuum is created by the jet stream drawing the following slurry forward into contact with the jet stream. In this manner the slurry is continuously pumped. To prevent cavitation of the jet stream, gas regulated at a pressure slightly above the vapor pressure of the liquid comprising the jet stream is emitted from bore 62 adjacent the discharge of the nozzle. The gas pressure prevents cavitation by maintaining a pressure in the vicinity of the jet stream which is above the pressure at which the liquid comprising the jet stream will vaporize.

A second modification of the pump is shown in FIG. 3. This modification describes a centrally oriented nozzle 64 in combination with the jet pump' The nozzle receives high pressure primary liquid from a suitable source (not shown). Surrounding the nozzle and in concentric alignment therewith is a cylindrical gas plenum 66 which receives gas from a supply pipe 68. A pressure regulator 70 is located within pipe 68 and is regulated by a presure sensing means 72 in gas plenum 66.

The nozzle and gas plenum are centrally located in a cylindrical conduit 74 which contains a slurry to be pumped. Although not shown, it is to be understood that the slurry is received into the conduit by a suitable transport means as indicated by arrows 76.

The nozzle discharges a liquid jet stream into conduit 74 at its discharge end 78. Gas is directed from a gas nozzle in the vicinity of the jet stream adjacent the discharge end 78 of the nozzle 64.

The operation of the pump is similar to the operation described for the first two embodiments. A high pressure liquid jet strea emerging from nozzle 64 impinges on the slurry in conduit 74 driving it forward. Simultaneously a vacuum created by the jet stream drives the slurry forward into the path of the jet stream. In this manner the slurry is continuously pumped.

Regulated gas emerging from gas nozzle 80 is directed in the vicinity of the jet stream adjacent the discharge end 78 of nozzle 64. The gas prevents cavitation by maintaining the pressure adjacent the jet stream above the vaporizing point of the liquid in the jet stream.

Thus, as seen from the above description, harmful cavitation occurring in the jet stream of the pump is eliminated by passing a gas in the vicinity of the vena contracta of the jet stream. The gas is regulated at a pressure slightly above the vapor pressure of the liquid comprising the jet stream for most eflicient pumping. The means to prevent cavitation is readily adaptable to a jet pump having either an annular, peripheral or centrally oriented high pressure primary liquid nozzle. These jet pumps are more eflicient since energy losses associated with cavitation are eliminated and Wear on the pumps is minimal.

It should be understood that although ordinarily the most beneficial result for preventing cavitation and providing maximum pumping capacity for all pump embodiments discussed above is introducing the gas at a pressure slightly above the vapor pressure of the jet streams liquid, the pressure utilized may be that between the vapor pressure of the jet streams liquid and atmospheric pressure without adversely affecting the pumping action of the pump.

Also since the gas being introduced has a direct effect on the suction of the pump, its pressure may be regulated to vary the suction, in addition to preventing cavitation,

to provide desired pressure, density and flow rate characteristics of the pumps discharge. For example, in pumping particulate solids it is frequently desirable to regulate the density of the solids being pumped since these solids have a direct effect on the pumps discharge pressure and velocity. By increasing the gas pressure introduced into the pump, the pumps vacuum or suction is also decreased which correspondingly effects the amount of solids drawn into the liquid stream and pumped. Thus, the density of the pumps discharge will be reduced increasing the pumps discharge pressure and velocity. In this manner, regulation of the gas pressure between the vapor pressure of the jet stream and atmospheric pressure not only prevents cavitation but also determines the discharge characteristics of the pump.

It will thus be seen that the objects set forth above, among those made apparent from the preceding description, are efiiciently attained and, since certain changes may be made in carrying out the above method and in the construction set forth without departing from the scope of the invention, it is intended that all matter con tained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

It is also understood that the following claims are intended to cover all of the generic and specific features of the invention herein described, and all statements of the scope of the invention which, as a matter of language, might be said to fall there-between.

Now that the invention has been described, what is claimed is:

1. A jet pump comprising a conduit for transporting a fluidized medium to be pumped, a nozzle communicating with the interior of said conduit, said nozzle being adapted to emit a highly pressurized liquid as a jet stream, which jet stream forms a vena contracta immediately downstream of said nozzle, and means for directing gas into the interior of said conduit at a location contiguous with the vena contracta downstream of the nozzle.

2. The jet pump of claim 1 further comprising regulating means for controlling the pressure of the gas to a pressure slightly above the pressure of the liquid in the jet stream.

3. The jet pump of claim 1 wherein said means for directing gas into the interior of said conduit is a pipe conto the atmosphere.

4. The jet pump of claim 2 wherein said regulating means is automatically controlled by a pressure sensor in communication with said means for directing gas into the interior of said conduit.

5. The jet pump of claim 2 wherein said regulating means comprise a fixed variable orifice for directing gas into the interior of said conduit.

6. The jet pump of claim 1 wherein said conduit comprises an inlet cylinder and a discharge cylinder, said inlet cylinder being of smaller diameter leading into said discharge cylinder, and said nozzle comprises the annular opening at the juncture of said inlet and outlet cylinders.

7. The jet pump of claim 1 wherein said nozzle comprises at least one bore through the wall of said conduit inclined in the direction of flow of the medium to be pumped.

8. The jet pump of claim 1 wherein said nozzle comprises a centrally oriented nozzle with respect to said conduit.

References Cited UNITED STATES PATENTS 655,615 8/1900 Evans 103-259 2,973,718 3/1961 Deutsch 103262 3,010,232 11/1961 Skakel et a1. 103-262 X 3,031,977 5/1962 Elliott 103258 X 3,448,691 6/1969 Frazier l03271 X 3,452,751 7/1969 Austin l03-258 X DONLEY J. STOCKING, Primary Examiner W. J. KRAUSS, Assistant Examiner 

